Introduction
Insect migrations and outbreaks of their offspring population are closely related to diverse adaptive strategies that enable migratory species to relocate from deteriorating habitats to suitable breeding habitats (Chapman et al., Reference Chapman, Reynolds and Wilson2015; Sappington, Reference Sappington2018). However, for many migratory species such as Spoladea recurvalis (Shirai, Reference Shirai2006), Mythimna separata (Jiang and Luo, Reference Jiang and Luo2005), and other lepidopteran insects (Nakasuji and Nakano, Reference Nakasuji and Nakano1990; Jiang et al., Reference Jiang, Luo and Sappington2010), not all individuals are destined to migrate. There is usually the co-existence of migrants with stronger flight capacities and residents with greater reproductive capacities. Such differentiations within populations are influenced by external environmental conditions or intrinsic physiological factors during development (Jiang et al., Reference Jiang, Luo, Zhang, Sappington and Hu2011; Dingle, Reference Dingle2014; Zhang et al., Reference Zhang, Cheng, Chapman, Sappington, Liu, Cheng and Jiang2020).
Temperature is a major environmental factor, which directly or indirectly affects insect population growth, development, and subsequent migratory behaviors, among other biological processes (Piyaphongkul et al., Reference Piyaphongkul, Pritchard and Bale2012; Pineda et al., Reference Pineda, Dicke, Pieterse and Pozo2013; Zhang et al., Reference Zhang, Rudolf and Ma2015b; Jiang, Reference Jiang2018). For example, exposure to extreme temperatures in an immature stage (i.e. egg, larval, or pupal stage) negatively affected survival, development, and fecundity of adults and even of subsequent offspring survival through trans-generational effects in some insect species (Foerster et al., Reference Foerster, Doetzer and de Castro2004; Klockmann et al., Reference Klockmann, Günter and Fischer2017; Chen et al., Reference Chen, Zhang, Qi, Xu, Hou, Fan, Shen, Liu, Shi, Li, Du and Wu2019a, Reference Chen, Zhang, Ma and Ma2019b). In some species, adults exhibited decreased mating success, impaired oocyte development, reduced sperm production and viability, when they were exposed to stressful temperatures (Rinehart et al., Reference Rinehart, Yocum and Denlinger2000; Gruntenko et al., Reference Gruntenko, Bownes, Terashima, Sukhanova and Raushenbach2003). Li et al. (Reference Li, Xu, Ji and Wu2018) demonstrated that both low and high temperatures had negative effects on reproduction across developmental duration and stage in M. separata. Similar results were reported from Ophraella communa (Zhou et al., Reference Zhou, Guo, Chen and Wan2010), Grapholitha molesta (Liang et al., Reference Liang, Zhang, Ma, Hoffmann and Ma2014), and other insect species (Yu et al., Reference Yu, Chi and Chen2013; Qin et al., Reference Qin, Zhang, Liu, Sappington, Cheng, Luo and Jiang2017). Insects have different thermal sensitivities which vary with their different life stages (Kingsolver et al., Reference Kingsolver, Higgins and Augustine2015), and exhibit trade-offs between reproductive traits and response to environmental impacts, which may alleviate the stress of extreme temperatures (Ma et al., Reference Ma, Ma and Pincebourde2020). In some migratory species, the first day after adult eclosion (within 24 h) is a critical period (Luo et al., Reference Luo, Li, Jiang and Hu2001; Guo et al., Reference Guo, Li, Zhang, Liu, Zhai and Gao2019a), during which low temperature can alter the behavior strategies between migration and reproduction (Zhang et al., Reference Zhang, Jiang and Luo2008b). However, little is known about the specific effects of the intensity, duration, and frequency of extreme low temperatures on newly emerged migratory insects.
Mythimna separata (Walker) is one of the most destructive agricultural pests in Asia and Oceania, with annual outbreaks in nearly 30 countries (Sharma and Davies, Reference Sharma and Davies1983; Lee and Uhm, Reference Lee, Uhm, Drake and Gatehouse1995). In China, this species seriously threatens crop production and has caused severe economic losses, especially in corn, wheat, and rice (Zeng et al., Reference Zeng, Jiang and Liu2013; Jiang et al., Reference Jiang, Li, Zeng and Liu2014). Like most migratory moths, M. separata exhibits a trade-off in energy allocation between migration and reproduction (Jiang et al., Reference Jiang, Luo and Sappington2010), which was defined as the ‘oogenesis-flight syndrome’ (Johnson, Reference Johnson1969). Migration is an energetically costly activity, and competition for limited internal resources may result in the suppression of reproductive development. However, migration-mediated reproductive costs are not absolutely negative for all life-stages, but instead depend on age, duration, and frequency of exposure of the moth's initial flight to environmental factors (Zhang et al., Reference Zhang, Pan, Sappington, Lu, Luo and Jiang2015a). To date, few studies have explored how newly-emerged migratory moths which encounter extreme low temperatures allocate energy between migration and reproduction (Zhang et al., Reference Zhang, Luo and Jiang2008a, Reference Zhang, Jiang and Luo2008b; Jiang et al., Reference Jiang, Luo, Zhang, Sappington and Hu2011). Furthermore, it is unclear whether low temperatures can induce the oogenesis-flight syndrome in this species.
To address these questions, we investigated the effects of exposing M. separata to low temperatures for various durations and at various ages, on the reproductive development and energy allocation in the laboratory. These results revealed periods at which adults are sensitive to low temperatures and reflected the complex roles of cold stimulation in the behavioral and physiological regulation of reproductive and ovarian development. This study provides valuable information for elucidating the M. separata population adaptations to extreme low temperatures and developing the potential control applications against this pest in China
Materials and methods
Insect rearing
The M. separata used in this experiment were collected from the Guilin area of Guangxi Province, China. The population was maintained for three generations under laboratory conditions (24 ± 1°C, 70% relative humidity (RH), and a photoperiod of 14:10 L:D). To obtain migrants, newly hatched larvae were reared in round glass bottles (9 cm × 13 cm, diameter × height) with fresh corn seedlings for pupation (ten larvae per bottle) as described in previous studies (Zhang et al., Reference Zhang, Luo and Jiang2008a, Reference Zhang, Jiang and Luo2008b). After emergence, male and female adults were paired (♂ + ♀) and reared in cylindrical plastic cages (10 cm × 20 cm, diameter × height). Fresh 5% (v/v) honey solution was provided daily until adult death (Jiang et al., Reference Jiang, Luo, Zhang, Sappington and Hu2011). Additionally, the instar stage and number of surviving larvae were examined daily. If the number of larvae was less than ten per bottle (because they had been eaten or lost), larvae of the same size were replaced to ensure a fixed density (ten larvae per bottle).
Low temperature treatments
To investigate the effects of low temperature on the reproductive ability and ovarian development, newly-emerged moths (within 24 h after eclosion) were exposed to three temperatures (6, 12, 18°C) for 24 h. Moths exposed to 24°C served as the control. The number of replicates of the control, 6, 12, and 18°C treatments was 26, 31, 28, and 31 pairs (♂ + ♀), respectively.
Following results obtained from the temperature sensitivity experiment, we chose 6°C (as this temperature significantly stimulated adult reproductive and ovarian development) for subsequent experiments to assess the duration of adults' sensitivities to low temperatures for regulating reproduction and development. Exposure to 6°C for four durations (0, 8, 16, 24 h) was carried out on 1-day-old adults. The time points for these durations were 8 h (19:00–03:00), 16 h (19:00–11:00), and 24 h (19:00–19:00). Each treatment had 30 replicates.
The third experiment was used to identify the age at which adults were sensitive to 6°C. Newly-hatched female and male moths were exposed to 6°C for 24 h (only this duration significantly accelerated adult reproduction and ovarian development) on days 1–4 after adult eclosion (1, 2, 3, and 4 days old). Moths exposed to rearing conditions as described above served as the control. Each duration or control treatment was replicated 30 times. In this study, all females and males were paired before the cold temperature exposure.
Flight capacity assessments
Flight capacity of female M. separata was conducted using a 48-channel flight mill system (Cheng et al., Reference Cheng, Luo, Jiang and Sappington2012). All tethered moths were attached to the round arm of the flight mill with 502 super adhesives. Three parameters (flight duration, flight distance, and average flight velocity) were automatically recorded for 12 h from 20:00 through 08:00 (an optimal period for studying M. separata flight behavior), as described in previous studies (Luo et al., Reference Luo, Jiang, Li and Hu1999; Jiang et al., Reference Jiang, Luo and Sappington2010; Zhang et al., Reference Zhang, Cheng, Chapman, Sappington, Liu, Cheng and Jiang2020). All tested moths were provided with fresh 5% (v/v) honey solution before the tether tests. Flight tests were performed under dark conditions, maintained at 24 ± 1°C and 80% ± 10 RH (conditions promoting maximum flight capacity of M. separata). The flight capacity of female moths was examined for 1–5 days after low temperature treatment (6°C for 24 h), and their replicates were 30, 29, 28, 26, and 27, respectively.
Determination of triglyceride content
To assess the effects of low temperature on energy allocation, the triglyceride content of female moths exposed to 6°C for 24 h was determined at 1, 2, 3, 4, and 5 days after exposure. Adults of the same age exposed to 24°C served as the control. More than 25 pairs (♂ + ♀) in each treatment group were dissected in this experiment. Triglyceride was extracted and separated from the thorax and the entire abdomen using the triglyceride assay kit E1003-2 (Beijing Applygen Technologies Institute, Beijing, China) following the method described by Guo et al. (Reference Guo, Li, Zhang, Liu, Zhai and Gao2019a). Briefly, the thorax and abdomen were weighed separately using XP6 electronic balance (0.001 mg, Mettler-Toledo AG, Switzerland) after removal of the wings, head, and appendages. Samples were placed into 1.5 ml grinding tubes, to which 1000 μl lysing solution was added for full grinding, and then placed in a hot water bath at 70°C for 10 min. After centrifuging at 12,000 rpm for 5 min, the supernatants were collected for triglyceride measurement. The samples were determined at 550 nm using a microplate reader (SpectraMaxM5, Molecular Devices, USA).
Reproductive and ovarian development parameters
All reproductive parameters, including the pre-oviposition period (POP), period of first oviposition (PFO), oviposition period, lifetime fecundity, female longevity, male longevity, mating frequency, and mating percentage, were measured following the methods described in previous studies (Jiang et al., Reference Jiang, Luo and Sappington2010; Cheng et al., Reference Cheng, Luo, Jiang and Sappington2012; Zhang et al., Reference Zhang, Pan, Sappington, Lu, Luo and Jiang2015a). All newly emerged moths from the prior exposure to low temperature were paired and fed as described above. The number of eggs laid per female and mortality was recorded daily to calculate the POP, PFO, lifetime fecundity, and oviposition period. Mating percentage and mating frequency were recorded by observing the presence and number of spermatophores in the females after death. The POP was measured as the period from adult emergence to the first oviposition. The PFO served as a reproductive index for measuring the synchronization of first oviposition, which was defined as the number of days between the minimal POP and a female's POP. For example, a mean PFO of 2 for a treatment group indicates that, on average, all females begin to oviposit for the first time 2 days after the first case of oviposition in the same treatment group. A lower PFO indicated more synchronization which resulted in higher larval density (Cheng et al., Reference Cheng, Luo, Jiang and Sappington2012; Zhang et al., Reference Zhang, Pan, Sappington, Lu, Luo and Jiang2015a). Ovary state was recorded after female death to determine ovarian development grades under a binocular microscope following the standard described by Jiang et al. (Reference Jiang, Qu, Xia and Zeng2009) and Chen et al. (Reference Chen, Zhang, Qi, Xu, Hou, Fan, Shen, Liu, Shi, Li, Du and Wu2019a, Reference Chen, Zhang, Ma and Ma2019b). All reproductive parameters were calculated based on all females (including mated and virgin females).
Data analysis
Binary logistic regression model was applied to examine the influence of low temperature on the reproduction of M. separata adults. Experimental studies have shown that there are three main factors affecting the reproduction, namely low temperature (including 6, 12, and 18°C), low temperature durations (including 8, 16, and 24 h), and exposure age (including 1, 2, 3, and 4 days old) as continuous variables. All reproductive parameters (including POP, PFO, lifetime fecundity, oviposition period, mating frequency, mating percentage, female and male longevity, and ovary data) were classified into two degrees for the responses, respectively: 1 (above the average value of all controls), positive; 0, negative (below the average value of all controls). Flight capacity and triglyceride contents among different treatments were analyzed using one-way analysis of variance followed by Tukey's honestly significant difference (HSD) test (P < 0.05) or independent samples t-tests (P < 0.05). All statistical analyses were performed with the SPSS software (version 22.0; SPSS, Chicago, IL, USA).
Results
Binary logistic-regression model on effects of low temperature on reproduction
Exposure of 1-day-old M. separata to different low temperatures (6, 12, and 18°C) for 24 h had significant effects on reproductive performances. Results from the binary logistic-regression model indicated a negative relationship between low temperature and lifetime fecundity (P < 0.05), but a positive relationship with PFO and female longevity (P < 0.05, Table 1). Females exposed to 6°C for 24 h produced more eggs than the moths in the 12 and 18°C treatments and the controls (Table 2). The PFO of females treated with 6°C also decreased compared to those in the 12 and 18°C treatments, indicating that this temperature significantly stimulated synchronized oviposition compared to the other treatments (fig. 1a). Similarly, female longevity increased with the increase in temperature (Table 2). However, there were no significant differences in POP, oviposition period, mating frequency, mating percentage, male longevity, and ovarian development grade among the different low temperature treatments (figs 2a and 3a, Table 2). Collectively, these results suggested that 6°C significantly accelerated the reproductive performance of M. separata females.
Period of first oviposition (PFO) of M. separata females exposed to different temperatures (a), to 6°C for different durations (b), and to 6°C at various ages (c) on day 1 after emergence.
Preoviposition period (POP) of M. separata females exposed to different temperatures (a), to 6°C for different durations (b), and to 6°C at various ages (c) on day 1 after emergence.
Ovarian development grades of M. separata females exposed to different temperatures (a), to 6°C for different durations (b), and to 6°C at various ages (c) on day 1 after emergence.
Results of binary logistic-regression model on the effects of low temperature on reproductive parameters for M. separata (n = 300)
Reproductive performances of M. separata females exposed to different low temperature regimes for 24 h on the day following emergence
All durations (8, 16, and 24 h) of exposure of 1-day-old females to 6°C significantly affected their reproductive development. Low temperature duration was negatively associated with POP and PFO (P < 0.05), but significantly associated with lifetime fecundity and ovarian development grade (P < 0.05, Table 1). Females exposed to 6°C for 24 h had a reduced POP (fig. 2b) and shortened PFO (fig. 1b) compared to the 6 and 12 h treatments and the controls, resulting in females starting to oviposit earlier and improving spawning synchronization. By contrast, cold-induced lifetime fecundity and ovarian development grade of females exposed to 6°C were higher than the 6 and 12 h treatments and the controls (Table 3, fig. 3b). No significant relationships were observed between low temperature duration and oviposition period, mating frequency, mating percentage, female longevity, and male longevity (Tables 1 and 3). Taking the above results together, females exposed to 6°C for 24 h showed a significant acceleration in reproductive development.
Reproductive performances of M. separata females exposed to 6°C for different durations during the day following emergence
There were significant positive effects of exposure age on POP and PFO (P < 0.05), but negatively affected the lifetime fecundity and ovarian development grade (P < 0.05, Table 1). Low temperature (6°C for 24 h) treatment on the 1-day-old females significantly decreased their POPs (fig. 2c) and PFOs (fig. 1c) compared to the other treatments. One-day-old females exposed to low temperature had greater lifetime fecundity and ovarian development grade than the other treatments (Table 4, fig. 3c). However, there was no significant relationship between exposure age and oviposition period, mating frequency, mating percentage, female longevity, and male longevity (Tables 1 and 4). These results showed that the reproductive abilities of female M. separata as a result of exposure to low temperature largely depended on age. Also the first day after eclosion was most likely to be the short critical period. Collectively, these results also suggested that exposure to 6°C for 24 h on the first day after eclosion significantly accelerated the adult reproduction.
Reproductive performances of M. separata females exposed to low temperature (6°C for 24 h) on the first to fourth day after emergence
Flight capacity of female moths after the low temperature treatment
Female's flight capacity decreased on the first to fifth day after moths exposures to 6°C (fig. 4). Flight duration, distance, and velocity were significantly decreased at the first day (duration: t 58 = − 2.275, P = 0.027; distance: t 58 = − 2.435, P = 0.018; velocity: t 58 = − 2.131, P = 0.037) and the second day (duration: t 56 = − 2.000, P = 0.050; distance: t 56 = − 2.687, P = 0.009; velocity: t 56 = − 3.451, P = 0.001) after the low temperature treatment compared to the controls (fig. 4). In the low temperature group, females at the first day after treatment had the lowest flight duration and flight distance, which were significantly less than at the fourth day and fifth day (duration: F 4, 145 = 5.147, P = 0.001, fig. 4a), and at the third day and fourth day (distance: F 4, 135 = 5.147, P = 0.001, fig. 4b), respectively.
Flight duration (a), flight distance (b), and flight velocity (c) of M. separata females on days 1–5 after low temperature (6°C for 24 h) treatment. The same letters represent no significant differences among female ages at the 5% level by Tukey's HSD test. ‘*’ or ‘**’ represents significant or highly significant differences between the low temperature treatment and the control by Student's t-test (P < 0.05 or P < 0.01); ‘ns’ indicates no significant difference.
Triglyceride content of M. separata females after exposure to 6°C for 24 h
Low temperature (6°C for 24 h) and age significantly affected the triglyceride content of female adults (fig. 5). The triglyceride content of female thorax exposed to 6°C significantly reduced on the first day (t 58 = − 2.822, P = 0.007) and fifth day (t 58 = − 2.059, P = 0.045) compared to that of unexposed females of the control group. Age had a significant effect on the thoracic triglyceride content of females exposed to 6°C, which ranged from 7.80 to 12.13 μm g−1 (F 4, 135 = 5.12, P = 0.001, fig. 5a). In the control group, the lowest thoracic triglyceride content in females was recorded on the fourth day, which was significantly less than that at the second day (F 4, 135 = 5.81, P < 0.001, fig. 5a).
Triglyceride content in thorax (a), abdomen (b), and the total body (c) of M. separata females on days 1–5 after low temperature treatment (6°C for 24 h). Different lowercase letters indicate significant differences by Tukey's HSD test at 5% level. ‘*’ or ‘**’ represents significant or highly significant differences between the low temperature treatment and the control by Student's t-test (P < 0.05 or P < 0.01); ‘ns’ indicates no significant difference.
Low temperature significantly reduced the abdominal triglyceride content of females on the first, second, and fourth day (first day: t 58 = − 3.23, P = 0.002; second day: t 54 = − 2.20, P = 0.032; fourth day: t 50 = − 2.36, P = 0.022, fig. 5b). The abdominal triglyceride content of females exposed to 6°C increased rapidly and reached the maximum on the third day, which was significantly higher than that on the first day (F 4, 135 = 2.54, P = 0.043, fig. 5b). In the control group, age had effects on abdominal triglyceride content of females similar to those recorded in females exposed to 6°C. Female moths sampled on the fifth day following exposure had the lowest triglyceride content and were significantly less than that on the third day (F 4, 135 = 2.76, P = 0.030, fig. 5b).
Likewise, the total triglyceride content significantly decreased on the first and second day (day 1: t 58 = − 3.71, P < 0.001; day 2: t 54 = − 2.16, P = 0.035, fig. 5c) after the low temperature treatment on the 1-day-old females. Age significantly affected the total triglyceride content of exposed females and unexposed females of the control group, similar to the abdominal triglyceride content (low temperature group: F 4, 135 = 3.70; df = 4, 135; P = 0.007; control group: F 4, 135 = 3.48, P = 0.010, fig. 5c).
Discussion
Extreme low or high temperatures negatively affect insect survival, reproduction, and population growth (Levie et al., Reference Levie, Vernon and Hance2005; Zhao et al., Reference Zhao, Hoffmann, Xing and Ma2017), caused by physical damage, proteins denaturing, or metabolic injuries (Turnock and Fields, Reference Turnock and Fields2005; King and MacRae, Reference King and MacRae2015). Insects tend to invest more energy to improve survival, but also pay a high cost by temporally delaying development and reducing reproduction output, in response to extreme temperatures (Potter et al., Reference Potter, Davidowitz and Woods2011; Zhao et al., Reference Zhao, Hoffmann, Xing and Ma2017). Development may rapidly return to normal rates following temporary exposure to low temperature, but the reproductive cost of this is not adequately known (Tezze and Botto, Reference Tezze and Botto2004). In this study, we demonstrated that exposure to 6°C for 24 h significantly accelerated reproductive and ovarian development in M. separata at no readily apparent reproductive costs. Our result was consistent with previous findings that moving adults from cold to warm temperature resulted in a marked increase in juvenile hormone (JH) biosynthesis (significantly higher than when held at constant high or cold temperature regimes), thereby accelerating the onset of sexual maturation (Cusson et al., Reference Cusson, McNeil and Tobe1990; Zhao et al., Reference Zhao, Feng, Wu, Wu, Liu, Wu and McNeil2009). Furthermore, the male pre-response period to female sex pheromone was determined to have significantly shortened after cold-stress the first day after eclosion for M. separata (Jiang et al., Reference Jiang, Luo, Zhang, Sappington and Hu2011), and in turn resulted in an increase in mating frequencies between females and males, which also increased the proportion of mated females with mature oocytes among populations (Coombs et al., Reference Coombs, del Socorro, Fitt and Gregg1993; Rhainds, Reference Rhainds2010).
Generally, migratory insects can escape from external environments including extreme temperature by migration, but long-range migration is clearly energetically costly, which is associated with increased risk of mortality (Arrese and Soulages, Reference Arrese and Soulages2010; Dingle, Reference Dingle2014; Chapman et al., Reference Chapman, Reynolds and Wilson2015). In many species, migratory behavior and flight system development consume much energy, resulting in the suppression of reproductive development during migration, according to the oogenesis-flight syndrome (Johnson, Reference Johnson1969; Lorenz, Reference Lorenz2007). Population differences in reproductive behavior and migration capacity influence insects' life history strategies to the environmental conditions experienced during development (Nijhout, Reference Nijhout1999; Jiang, Reference Jiang2018). In M. separata and many other migratory species, residents showed stronger reproductive performance and poorer flight capacity (Luo et al., Reference Luo, Li, Cao and Hu1995; Jiang and Luo, Reference Jiang and Luo2005; Shirai, Reference Shirai2006; Wang et al., Reference Wang, Zhang and Zhai2010). Exposure to low temperature of different intensities and durations had reproductive consequences in the form of shortened POP, increased lifetime fecundity, synchronized reproduction, and ovarian development level in M. separata females. Furthermore, longevity was not decreased for either females or males in response to low temperature exposure. These results suggest that exposure to 6°C for 24 h may be beneficial for reproduction, at little-to-no reproductive cost. Therefore, we hypothesize that low temperature can trigger a behavior shift from migrants to residents in a short sensitive period (the first day after adult eclosion). Binary logistic-regression model had also demonstrated that exposure to 6°C for 24 h on the first day after eclosion had a significant acceleration in adult reproduction.
Interestingly, in some migratory insects, there is an environmentally sensitive period for altering the direction of insect development (Duan et al., Reference Duan, Weber and Dorn1998; Bertuso et al., Reference Bertuso, Morooka and Tojo2002). Our results showed that the first day after eclosion is the critical period for M. separata to respond to low temperature, which was consistent with previous findings (Zhang et al., Reference Zhang, Luo, Jiang and Hu2006, Reference Zhang, Jiang and Luo2008b). Guo et al. (Reference Guo, Li, Zhang, Liu, Zhai and Gao2019a) reported that starvation during day 1 after eclosion prolonged the POP and promoted the onset of migration in Cnaphalocrocis medinalis. However, starvation significantly reduced POP, dry weight of flight muscle, and flight capacity of M. separata females, causing a shift from migrant adults to residents (Zhang et al., Reference Zhang, Luo and Jiang2008a). Zhang et al. (Reference Zhang, Cheng, Chapman, Sappington, Liu, Cheng and Jiang2020) further found that JH titers regulated the transition on the first day post-emergence in M. separata. Additionally, exposure to low temperatures significantly decreased the PFO of 1-day-old females compared to that of the control, indicating that stimulated synchronized oviposition resulted in the increase in subsequent larval densities and even population outbreak, similar to Spodoptera exigua (Jiang et al., Reference Jiang, Luo and Sappington2010), Loxostege sticticalis (Cheng et al., Reference Cheng, Luo, Jiang and Sappington2012) and C. medinalis (Zhang et al., Reference Zhang, Pan, Sappington, Lu, Luo and Jiang2015a).
Flight capacity of newly-emerged females on the first day and second day decreased significantly after adults' exposures to low temperature, while reproduction significantly increased. These results show that there exists a trade-off between flight and reproduction when adults are faced with low temperature. Insect behavioral strategies result from physiological processes, especially the redistribution or transformation of intrinsic energy, causing a shift between reproduction and migration (Auerswald and Gäde, Reference Auerswald and Gäde2000; Guo et al., Reference Guo, Yang, Li, Liu and Zhai2019b). Triglyceride is a major fuel source for supporting long-distance in migratory insects (Murata and Tojo, Reference Murata and Tojo2004; Li et al., Reference Li, Gao, Luo, Yin and Cao2005). Its content variations in the thorax and abdomen thus result in the transition between migration and reproduction (Guo et al., Reference Guo, Li, Zhang, Liu, Zhai and Gao2019a). In this study, low temperatures significantly affected the changes in the triglyceride contents of M. separata females. Abdominal triglyceride content and the total content of females exposed to lower temperatures were all lower than the controls, suggesting that less energy sources were used to maintain long-range migration, resulting in weakening their migratory propensity. Furthermore, the thoracic triglyceride content of females also decreased compared to the controls, indicating the use of less energy in flight-readiness, which delayed the development of flight muscle. These results suggest M. separata allocates more energy into breeding, but reduce energy supply for migration in response to temporary low temperature. This speculation is in agreement with previous studies that reported that, there is a trade-off in energy-allocation between reproduction and flight (Rankin and Burchsted, Reference Rankin and Burchsted1992; Jiang et al., Reference Jiang, Luo and Sappington2010). Similarly, male moths also engage in a trade-off between rapid flight initiation and suboptimal flight performance (Crespo et al., Reference Crespo, Goller and Vickers2012). Male moths are believed to bear the major costs of finding a mate during pheromone-mediated upwind flight (Thornhill and Alcock, Reference Thornhill and Alcock1983). Crespo et al. (Reference Crespo, Vickers and Goller2014) reported that Helicoverpa zea males sensing the female pheromone at low ambient temperatures (6°C mean difference with cold treatment) took off with lower thoracic temperature, shiver for more time, and warm up slower than males tested at higher ambient temperatures. Thus, males that experienced cold-stress consumed more energy resources to successfully locate and mate with a calling female, resulting in the reduction of their subsequent flight performances. Taken together, our data indicate that low temperatures trigger the shifting from migration to reproduction in M. separata and exhibit the oogenesis-flight syndrome via regulating the energy redistribution.
Shifting from migrants to residents has great adaptive significance in M. separata. Jiang et al. (2000) reported that roundtrip migration of M. separata was an adaptive life history strategy to escape from high temperatures of spring and summer in South China and low temperatures of fall and winter in North China. Cold tolerance of this species determines its overwintering range in the area south of 33°N latitude, where the average temperature in January is above 0°C (Li, Reference Li and Luo1961). Zhang and Li (Reference Zhang and Li1985) have demonstrated that low temperatures below 11°C greatly reduce the flight capacity of M. separata adults, and their flight activities completely stop when ambient temperature is at 5°C. Jiang et al. (Reference Jiang, Cai, Luo, Cao and Liu2003) reported that M. separata emigrants can take off only when the ground temperature near dusk is at least 8–10°C, indicating that adults cannot take off and migrate at a temperature ≤8°C. Additionally, adults emerging in the spring are most likely to encounter low temperature stimulation, thereby inhibiting pre-migratory feeding for migrants and resulting in the shifting from migrants to residents during the sensitive stage. This shift is consistent with underlying physiological changes related to starvation (Zhang et al., Reference Zhang, Luo and Jiang2008a). Hence, we posit that temperatures lower than 6°C may further exacerbate the shift from migrants to residents during the sensitive stage. However, when there is no extreme deterioration in environmental conditions on the first day of adult life, M. separata adults tend to migrate to new habitats for offspring. But when adults are exposed to a stressful environment during the sensitive stage, shifting away from migratory behavior in favor of resident behavior may be beneficial during the sensitive stage (Zhang et al., Reference Zhang, Luo and Jiang2008a). Migrant and resident polytheisms in M. separata are adaptations to environmental variations (Zhang et al., Reference Zhang, Cheng, Chapman, Sappington, Liu, Cheng and Jiang2020). A portion of M. separata adults remain and reproduce in the local habitat, which may represent an adaptive ecological strategy adopted to cope with extreme environmental temperatures (Zhang et al., Reference Zhang, Jiang and Luo2008b). This strategy may decrease the proportion of migratory individuals in populations, thereby alleviating damage to crops in new breeding grounds and reducing difficulties in accurately predicting population infestations and outbreaks of M. separata.
In conclusion, we report that exposure of adult M. separata to 6°C for 24 h contributes significantly to their reproductive and ovarian development during a short critical period (the day after eclosion), and this could be an adaptive strategy for coping with low temperature environment, thereby switching from migratory behavior to resident behavior in M. separata. These findings highlight the importance of low temperature in modulating insect developmental pathways and also aid in the development of new strategies for controlling M. separata populations.
Acknowledgements
This research was supported by the Fundamental Research Funds of China West Normal University (Project No. 412/412834). We would also like to acknowledge the anonymous reviewers for their constructive and valuable comments.
Conflict of interest
None.
